@Rafaelrgm, the reason why Marie's route is more interesting is because it's not as fragmented and chaotic as the main story which is focuses on too many things that are constantly happening in too many places and all this without completing any of them for good until the epilogue as there is no brakes on the death flag train, almost all of them were triggered by Marie's act of smashing the main route into smitheries and consequences of Leon's fixing them before the country/world were destroyed.
In the side-story Leon's retirement plan gone just as planned and things he done happened at his own pace of background mob character and from his own choices without "oh crap, I need to do something fast" each few days when some new crap was triggered by Marie or her harem and was about to kill himself in the process. In the main story it gone astray from the very start and Leon needed to fix things which caused him to become a protagonist of sorts, do many things that he didn't wanted and become too flashy while his "sweet peaceful home" was destroyed by descending meteor which consisted of protagonist+villainess that were torn out of main story like some garbage and spiced up by a promotion.
In the side-story we have a similar main characters with common grounds of being reincarnators with tough past/similar initial circumstances in this world/some incestuous vibes of seeing a sibling in each other, the story is fully focused on them and not much unexpected things happen so the story goes at slow pace, even if something happens it makes ther bond stronger. Marie is way more honest and didn't destroyed the main route so it works like planned, Leon didn't shown his strength too much, didn't triggered too many events, girls from the main route aren't interested in him(in process of budding lol?), "demonic" Leon almost didn't had the chances to show up with his iconic "legitimate self-defence till your total destruction". In the main story we have a duke's daughter and future saint being dropped on the lower noble character that wanted just to live peacefully and marry some knight's daughter, his peaceful future was destroyed by the reincarnator that messed with the main route and there will be even more of them, let's not forget those idiotic capture male targets that make things even more messy. To fix it Leon needs to break that rotten reverse harem so he used Ange's dilemma of not having someone to fight for her sake in the duel to beat the crap out of the reverse harem, raising affection points, before he also gave some food for the stray cat(Olivia) which also unnecessarily raised affection points. His actions ended with promotion, girls in academy that seen men as trash before now seeing him as enemy or problematic candidate so Leon is unable to find a fiance, nobles seeing him as a dangerous upstart noble and start to mess with him which only promoted him even more and showed his battle potential which triggered nobles to mess with him even further, having two VERY problematic girls being interested in him. Too many chaotic things happen which is a good and interesting for action story with elements of romcom which goes to nowhere but it's too flashy and bad thing for a pure romance as there is no time for it.
Having a few routes in one tankoubon with some differences and additional plot is a pretty good thing, especially when it isn't so realistic to publish some "light" and "dark" series of the same title as tankoubons. It's hard to make the amount of side-story big enough to complete a book but how it's implemented here isn't that bad for both routes - making both of series big will ends up with repeating the most of basic plot to make it readable as independent book, or you'll have a 1-2 volumes of compressed side-story that is absolutely unreadable without knowing the plot of main 7-8 volumes. Here we have the same main characters and not some side characters to have an excuse to make spin-off about them, also showing additional information from the POVs of main chapters in the "different" storyline looks many times more harmonious than adding many POVs chapters from random characters, latter kills such a stories by making them even more messy and chaotic. There is no real need to have too much of plot alterations than we have in Marie's route already so it's pretty enough to add a few chapters in each "plump" tankoubon to have the same 1-2 "virtual dark volumes" for the whole series.
I liked how that "route thing" was done in the anime Amagami SS - even while each heroine's route/arc had just 4 episodes, they didn't felt being too short as there were many things between those routes that helped to complete the shallow parts of each of them, highlighted the basic storyline while the character development and backstory was shown by using different perspectives of each route/arc and even if some parts of personality or plotline weren't shown in their own routes, they were shown in other ones and thus completed it. As for me, what Marie's route done even while being short is still enough to be seen just as "full" route - it's uses the same world with few different choises and there is only one heroine just like in Amagami SS, and even if a few new girls/chooses will be added later, their backstory is already highlighted enough in the main story and doesn't need to be repeated, there wouldn't be "the heck is that, the heck is this, the heck is you?" when you don't understand what happened or it will need too much of text to be explained.